Sand fiddler crabs are a common sight on the beaches of the eastern U.S. The small creatures (typically about an inch wide, not including their legs) are distinguished by the unusual asymmetry exhibited by the claws of the males – one claw grows to almost grotesque proportions, longer than the body of the crab itself.
The males use their claws to defend their breeding burrows from other males, and as an advertisement to females looking for mates – they stand at the opening to their burrows, waving their giant claws in circles, trying to entice female crabs to join them.
In areas with a semidiurnal tide regime, sand fiddler crab mating tends to peak during spring tides, which occur when the moon is full or new – when the gravitational forces between the sun, the moon, and the earth line up to create higher high tides and lower low tides, and the ‘tidal flux’ (the difference between the high tide and the low tide) is especially large. The sand fiddler crab larvae that result from mating during a spring tide are released during the next spring tide, when that greater tidal flux can help the larvae survive.
A group of researchers working on the western coast of Florida recently investigated whether or not male sand fiddler crabs’ claw-pinching strength follows the lunar cycle to peak during spring tides, too.
Their results, published in the Journal of Experimental Marine Biology and Ecology, suggest that male sand fiddler crab strength and behavior does correspond to the lunar cycle. The scientists found that crabs who were courting females and defending burrows (which were located in relatively high and dry areas without much food nearby) had more force behind their claw-pinches than crabs roaming around in large groups, or droves, looking for something to eat, and that both the number of courting males and the males’ claw strength peaked during new and full moons.
Courting females and defending burrows appear to be energetically costly activities, and male crabs periodically took a break from mating activities to restore their strength by eating; as the scientists note, “claw power declined during courtship and increased while droving.” In general, the strongest male sand fiddler crabs were able to synchronize their eating-and-mating cycle with the lunar and tidal cycles, allowing them to stand outside their burrows waving their claws during the times when they were most likely to successfully mate.